Resolving nanostructure and chemistry of solid-electrolyte interphase on lithium anodes by depth-sensitive plasmon-enhanced Raman spectroscopy

The solid-electrolyte interphase (SEI) plays crucial roles for the reversible operation of lithium metal batteries. However, fundamental understanding of the mechanisms of SEI formation and evolution is still limited. Herein, we develop a depth-sensitive plasmon-enhanced Raman spectroscopy (DS-PERS) method to enable in-situ and nondestructive characterization of the nanostructure and chemistry of SEI, based on synergistic enhancements of localized surface plasmons from nanostructured Cu, shell-isolated Au nanoparticles and Li deposits at different depths. We monitor the sequential formation of SEI in both ether-based and carbonate-based dual-salt electrolytes on a Cu current collector and then on freshly deposited Li, with dramatic chemical reconstruction. The molecular-level insights from the DS-PERS study unravel the profound influences of Li in modifying SEI formation and in turn the roles of SEI in regulating the Li-ion desolvation and the subsequent Li deposition at SEI-coupled interfaces. Last, we develop a cycling protocol that promotes a favorable direct SEI formation route, which significantly enhances the performance of anode-free Li metal batteries.

study SEIs. Whereas, this situation can be circumvented by borrowing LSP-active SHINs introduced on flat Cu surface to enhance the Raman signals of SEIs grown on Cu surface. For this system, there are only two types of hotspots located at the junctions of flat Cu-to-SHINs and SHIN-to-SHIN that in principle enhance the Raman signals of SEI in the inner and outer region, respectively. The measured bands associated with SEI components are highly complex and dynamically varying, especially at the initial stage of SEI formation. Therefore, to obtain higher detection sensitivity for SEIs, the flat Cu with SHINs is replaced by the SERS-active nanostructured Cu created by electrochemical ORC method. Three different type of hotspots can be formed in the junctions of Cu-to-Cu (I), Cu-to-SHIN (II) and SHIN-to-SHIN (III), respectively, to form a more active integrated plasmonic enhancement substrate with unique depth sensitivity. In the early stage of SEI formation, the hotspots (I) and (II) enhance the Raman signals of SEI in the inner region. During the growth of SEI, hotspots (II) and (III) act to detect the chemical structure of SEIs including outer regions. In this circumstance, the plasmonic coupling is strengthened and further enhanced Raman signals of SEI can be detected.
It is noted that the application of 3D current collector is one of the conventional strategies to improve the performances of Li metal anodes [2][3][4] . And the nanostructured Cu surface with slightly increased surface area as well as inhomogeneous charge distribution does not seem to significantly affect the SEI chemistry, at least on a macroscopic scale, as evidenced by similar

S3. The finite-element method simulation of optical electric-field distribution at initial integrated Cu-SHINs substrate
The electric field distribution and enhancement from initial integrated Cu-SHINs substrate was modelled using a finite-element method (FEM). The structure selected for modelling was described in Supplementary Methods. Due to the potentially broad size distribution of the nanostructured Cu surface obtained by electrochemical ORC method, we have modelled the electric-field distribution for the integrated Cu-SHINs substrate with both larger and smaller Cu islands. As illustrated in Supplementary Fig. 3, in both cases, the regions of enhanced field strength (i.e. hotspots) are formed under laser excitation in three types of junctions, namely Cu island-to-Cu island, Cu island-to-SHIN and SHIN-to-SHIN, and their average plasmon enhancement factors (EFs) exceed 8 orders of magnitude (with 785 nm laser), which is theoretically strong enough for ultrasensitive detection of SEI components. In principle, in the early stages of SEI formation, the hotspots located in the junctions of Cu-to-Cu and Cu-to-SHIN enhance the Raman signals of SEI in the inner region. During the growth of SEI, hotspots located in the junctions of Cu-to-SHIN and SHIN-to-SHIN act to detect the chemical structure of SEIs including outer regions. This simulation indicates that the integrated Cu-SHINs substrate with multi-hotspots can serve as a favorable surface plasmon-enhancement system for bottom-up probing the formation and evolution of SEIs in real-time even with the presence of the background electrolyte.

SHINs substrate
In the case of traditional solid-liquid interface, it is relatively easy to perform SERS/SHINERS analysis of general adsorbates in the well-controlled nanogap between nanoparticle and substrate surface, since their position on the substrate remain largely constant during the electrochemical processes. However, the situation is totally different in the solid-solid interface such as the metal-SEI interface under investigation in this work, in which SEI grows in vertical direction and changes dynamically, thereby affecting the local electromagnetic field distribution and the plasmon enhancement performance of the integrated Cu-SHINs substrate. It is noteworthy that the presence of SHINs introduces a shielding effect for electrolyte reduction and thus SEI growth in the gap between SHINs and metal surface. Therefore, SEI formation is initiated only at the metal surface, and the growth of SEI is expected to leave the SHINs partially embedded in the SEI ( Supplementary Fig. 4). Due to the fact that Raman signals decrease exponentially with increasing distance, the possibility of SEI growth in the gap between SHINs and metal surface (i.e. beneath the SHINs so that SHINs float on the SEI) can be excluded because under such a circumstance Raman signals of SEI components could not be detected efficiently, which is contradictory to our experimental results.
To investigate the influence of SEI growth on the electromagnetic field distribution, FEM simulations were carried out on the Cu-SHINs substrate covered by different thickness of SEI ( Supplementary Fig. 5). It is seen that the growth of SEI does not weaken the electromagnetic field strength of the coupled plasmonic substrate, and the average EF of this configuration can still be as high as 10 10 , which is similar to that of bare integrated substrate. This is because that the SEI components, such as LiF, Li2O, LiOH, etc., absorb light weakly and their refractive indexes are very close to that of the electrolyte 1 . It may be necessary also to note that the thickness of SEIs on Li metal anodes typically ranges from several nanometers to tens of nanometers, which would not completely encapsulate or warp up a ~60 nm diameter Au@SiO2 nanoparticle. Therefore, the enhanced Raman signals of SEI components arising from different depths of the inner and outer layer are achieved by hotspots located at the junctions of Cu islandto-Cu island, Cu island-to-SHIN and SHIN-to-SHIN, respectively, during SEI growth.

S5. The influence of Li deposition on the electromagnetic field distribution in the integrated Cu-SHINs substrate
In principle, Li is an s-electron metal and expected to provide strong LSPR effect. After Li deposition on the integrated Cu-SHINs substrate, the deposited Li would synergize with SHINs and become new SPR active sources to strengthen the plasmonic response and enhance Raman signals of SEI evolution. However, Li deposition has to proceed beneath a SEI, i.e. Li ion has to transport through SEI, which is then followed by nucleation and growth at metal/SEI interface. This could lead to two types of changes in the distribution of SHINs on the substrate upon Li growth, namely SHINs floating on and embedded in Li deposits, respectively, which has a significant impact on the electromagnetic field distribution. Note that SHINs are embedded in SEI in both cases as discussed above in Supplementary Note 2. To examine the electromagnetic-field distribution in the two cases, FEM simulations were performed and results are shown in Supplementary Fig. 6. It can be clearly seen that the electromagnetic field enhancement is weak to probe SEI when the SHINs are embedded in Li deposits, since most of hotspots are buried inside the Li deposits. On the contrary, nanoparticles floating on Li deposit can generate effective hotspots located at the junctions of Li-to-Li, Li-to-SHIN and SHIN-to-SHIN and thus still maintain the high enhancement factor of up to 10 10 , which enables sensitive detection of Raman signals of SEI components.

S6. Chemical restructuring of SEI with participation of metallic Li
The SEIs formation can proceed chemically and/or electrochemically depending on reduction conditions and participation or not of active Li metal. On the Cu surface, SEIs are mainly formed by electrochemical reduction of anion and solvent molecules, and their exact composition are dictated by electrode potential during the potential-dependent formation processes. Taking electrolyte of LiTFSI/DME-DOL as an example, upon negative shifting of the potential, higher oxidation state species, such as Li2SxOy, ROCO2Li, Li2NSO2CF3 would be formed and distribute non-uniformly in the vertical direction but mainly in the inner region.
This lay basis for a primary Cu-SEI, which is less stable and more conductive though f, TEM images Au@SiO2 nanoparticles after discharge. Scale bar, 50 nm.
We used cyclic voltammetry to check that the synthesized Au@SiO2 core-shell nanoparticles with ultra-thin shells are really pinhole-free. As shown in Supplementary Fig. 2b, the black curve reveals typical feature of bare Au nanoparticles with a characteristic reduction peak of Au surface oxide at about 0.9 V, whereas the red curve shows the feature of Au@SiO2 nanoparticles without the characteristic reduction peak, indicating the pinholes-free character of the ultra-thin SiO2 layer. Furthermore, the electrode with Au@SiO2 nanoparticles have already displayed very small charging/discharging currents of the electrochemical double layer capacitance, which also indicates the inert surface electrochemical property of the thin SiO2 shell. In addition to the electrochemical examination, we further used SERS method with pyridine as typical probing molecule for more sensitively checking whether the SiO2 shell are pinhole-free. As shown in Supplementary Fig. 2c, there are no signals related to pyridine from Au@SiO2 nanoparticles on Si wafer, which confirms the compactness of the ultra-thin SiO2 shell coated on Au core nanoparticles. Additionally, we should mention that SiO2 shell may have a potential risk of lithiation but highly depends on its size, crystallinity, morphology, and oxygen content. To inspect whether the SiO2 shell in our system goes through the lithiation process, we conducted TEM characterization for discharged Au@SiO2 nanoparticles.
Specifically, we first performed galvanostatic discharge measurement on Cu-SHIN substrate in DOL-based electrolyte ( Supplementary Fig. 2e). Then, the discharged Cu-SHIN substrate was washed with DME to remove residual electrolyte and put into DME solution for 1 h of ultrasonic cleanout, and finally the fallen Au@SiO2 SHINs were collected for TEM imaging.
As shown in Supplementary Fig. 2f, the thickness of SiO2 shell are virtually unchanged after discharge, compared to the initial SiO2 shell with thickness of ~2 nm, and no volumetric strain induced by lithiation can be detected. The above-presented experiments suggest that the SiO2 shell in our system does not undergo the lithiation process (or if it takes place, it is too inconspicuous to interfere with the experiments).  Supplementary Tables 2 and 3. Supplementary Fig. 13 shows an in-situ Raman measurements for SEI evolution conducted in real-time with precise control of applied potential. It is clear that the broad band at ~1050 cm -1 emerges first and gradually intensifies after Li OPD, which indicates species of Li2SxOy and ROLi are first generated from their higher oxidation state counterparts such as Li2NSO2CF3 due to the chemical reactions with metallic Li. As the potential become more negative and with extension of time, broad bands at low wavenumber emerge gradually, which are ascribed to the further decompostion of higher oxidation state species to lower oxidation state ones such as To investigate the influence of primary Cu-SEI on the follow-up Li-SEI, we further performed potential-dependent Raman measurements on the Cu-SHINs coupled plasmonic substrate in the same electrolyte system as shown in Supplementary Fig. 19. Prior to Li OPD, several individual bands appear during the negative potential excursion, whose vibrational assignment and principal chemical components are summarized in Supplementary Table 5. These bands are mainly from organic B-containing species as well as CO2/CO3 species, again confirming the organic feature of Cu-SEI in this case. Once Li OPD occurs, the broad bands associated with Li-SEI show up immediately, and their frequency and intensity are similar to those on the spectra in Fig. 3a in the main text. Especially, the band at 1850 cm -1 attributed to the symmetric stretching of C≡C bond from Li2C2 appears to be much strongly related to the one in Fig. 3a in the main text. It is noteworthy that there exist two different insights in the literature works regarding the mechanism for the appearance of Li2C2 species: Naudin et al. suggested the appearance of Li2C2 as a result of local degradation of organic species by laser-induced heating processes 5 ; while Schmitz and co-workers believed that the band is an intrinsic species from SEI 6 . In either case, the presence of this band indicates that the SEI contains significant amounts of organic components. It is likely that the longer Cu-SEI proceeding, the more high-oxidation state organic components in final Li-SEI, especially in the outer layer.  Fig. 26 show the proposed reaction mechanism for decomposition of LiDFOB and solvents, which can be electrochemically-driven and/or chemically-driven depending on whether it initiates on the current collector or reactive Li metal surface. For the sequential formation route, the reduction of DFOB anion initiates on the Cu surface and undergoes a partial ring-opening reaction (Fig. 4i in the main text) in relatively high potential region, which induces the B-O band cleavage and thus generates electron-deficient difluoroborate species. Meanwhile, the reduction of DEC coupled with a ring-opening reduction of FEC ( Supplementary Fig. 25) allows for the generation of electron-rich Li alkoxides (ROLi) or Li carbonates (ROCO2Li), which can subsequently react with difluoroborate. For the direct formation route, DFOB anions and solvents prefer capturing more electrons from reactive Li metal, which are then used for ring-opening reaction and thus produce electron-deficient products as well as low-oxidation state species such as LiF. In particular, reactions and polymerizations of the electron-deficient difluoroborate species with electron-rich ROLi or ROCO2Li could be accelerated via fluorine/oxygen exchange reaction on boron with plenty supply of metallic Li, which is conducive to generation of more LiF as well as polymeric components that consist of Bcontaining species like -B(OCH2CH2)n- (Fig. 4j in the main text). It has been a general consensus that SEI bearing polymeric-like structure composed of oligomers incorporated with inorganic species are usually elastic and more facile for Li ion conduction, which are beneficial to suppressing dendrite growth for uniform Li deposition. The proposed mechanisms are consistent with our experimental observations and provide some insights into the advantages of direct formation route with participation of metallic Li in improving cell performance.

Supplementary Figure 28 | Electrochemical performances of anode-free electrodes
covered by different SEIs. Specific discharge capacity versus cycle number and the corresponding Coulombic efficiency of the coin-type Cu‖NMC532 cells with Cu electrodes covered by different SEIs. These cells were cycled between 3.6 V and 4.5 V at the C/5 rate for charge and the C/2 rate for discharge. A 1C rate corresponds to a current density of 170 mA g −1 based on active NMC cathode material. Before the measurements, all the cells were subjected to two formation cycles at C/10 rate for both charge and discharge processes. It is demonstrated that a high average Coulombic efficiency (≈99.7%) as well as a high capacity retention 80% after 80 cycles can be achieved for Cu‖NMC532 cell with Cu electrode covered by directly formed SEI.